If you navigate to
'Dirty Bomb\ShooterGame\CookedPC\Maps'
you can see the maps that are in the game. If you copy one of the map folders, and rename everything (The folder, and everything in the folder) to anything you want. Go in to the game and press the tilda key (`), and type in 'switchlevel [The name of your 'map']. It goes into the map that you copied. So, in theory, if you could make a map that is compatible, you could import the map into the game. So.. when can we have custom maps. I dont think this would be that much work for SD, since all they need to do is release their map development software (the customized version of UE3). Please make this happen!

they said making an SDK is a lot of work

If they are making maps.. They would have an SDK, right? So, it would not be that much work, I think we may see an SDK being released in the near future. I would really like to see it cause, there are very talented people in this community. It would make map development much easier for SD.

No!, phantom should be the last merc you buy, he is not useful at all in competitive and he is only good when playing against people who are below level 10. He is quite hard to play because experienced players will see him even if he is using the cloak. I would recommend buying Pheonix, Bushwaker, Proxy or arty rather than him. Try him out next time he is on free rotation, if you like him, you should buy him.